T Follicular Regulatory Cells and Antibody Responses in Transplantation

Correlation of cTfh cells and memory B cells with AMR after renal transplantation

Renal transplantation is the preferred treatment option for patients with end-stage renal disease (ESRD) in a clinical setting. Antibody mediated rejection (AMR) is one of the leading causes of graft dysfunction. To address the current shortcomings in the early diagnosis and treatment of AMR in clinical practice, this article analyzes the distribution of different circulating T follicular helper (cTfh) cell subtypes and B cell subpopulations in peripheral blood and detects the cytokine levels of chemokine ligand 13 (CXCL13), interleukin-21 (IL-21), and interleukin-4 (IL-4) related to cTfh cells in peripheral blood of kidney transplant recipients. Moreover, we also explore the correlation between cTfh cells, peripheral blood memory B cells, and AMR, their value as early predictive indicators of AMR, and explore potential therapeutic targets for AMR patients. Our results indicate that the proportion of cTfh cells increased at the onset of AMR, which plays an important role in antigen-specific B-cell immune regulation. Activation of cTfh cells in AMR patients correlates with phenotypes of memory B cells and plasma blasts. cTfh cells and memory B cells have promising diagnostic efficacies and predictive values for AMR. The proportion of cTfh cells to CD4+ T cells and the proportion of memory B cells to CD19+ B cells are correlated with serum creatinine levels, indicating that cTfh cells and memory B cells may be involved in the progression of AMR. In addition, the CXCL13, IL-21, and IL-4, which were associated with cTfh cells, may be involved in the onset of AMR.

An Immune Atlas of T Cells in Transplant Rejection: Pathways and Therapeutic Opportunities

Short-term outcomes in allotransplantation are excellent due to technical and pharmacological advances; however, improvement in long-term outcomes has been limited. Recurrent episodes of acute cellular rejection, a primarily T cell-mediated response to transplanted tissue, have been implicated in the development of chronic allograft dysfunction and loss. Although it is well established that acute cellular rejection is primarily a CD4 + and CD8 + T cell mediated response, significant heterogeneity exists within these cell compartments. During immune responses, naïve CD4 + T cells are activated and subsequently differentiate into specific T helper subsets under the influence of the local cytokine milieu. These subsets have distinct phenotypic and functional characteristics, with reported differences in their contribution to rejection responses specifically. Of particular relevance are the regulatory subsets and their potential to promote tolerance of allografts. Unraveling the specific contributions of these cell subsets in the context of transplantation is complex, but may reveal new avenues of therapeutic intervention for the prevention of rejection.

Low switched memory B cells are associated with no humoral response after SARS-CoV-2 vaccine boosters in kidney transplant recipients

Introduction

The humoral response after SARS-CoV-2 vaccination and boosters in kidney transplant recipients (KTRs) is heterogeneous and depends on immunosuppression status. There is no validated immune measurement associated with serological response in clinical practice. Multicolor flow cytometric immunophenotyping could be useful for measuring immune response. This study aimed to study B- and T-cell compartments through Standardized EuroFlow PID Orientation after SARS-CoV-2 vaccination and their association with IgG SARS-CoV-2 seropositivity status after two doses or boosters.

Methods

We conducted a multicenter prospective study to evaluate humoral response after SARS-CoV-2 vaccination in KTRs. Heterologous regimen: two doses of inactivated SARS-CoV-2 and two boosters of BNT162b2 mRNA (n=75). Homologous vaccination: two doses of BNT162b2 mRNA and one BNT162b2 mRNA booster (n=13). Booster doses were administrated to KTRs without taking into account their IgG SARS-CoV-2 seropositivity status. Peripheral blood samples were collected 30 days after the second dose and after the last heterologous or homologous booster. A standardized EuroFlow PID Orientation Tube (PIDOT) and a supervised automated analysis were used for immune monitoring cellular subsets after boosters.

Results

A total of 88 KTRs were included and divided into three groups according to the time of the first detected IgG SARS-CoV-2 seropositivity: non-responders (NRs, n=23), booster responders (BRs, n=41), and two-dose responders (2DRs, n=24). The NR group was more frequent on mycophenolate than the responder groups (NRs, 96%; BRs, 80%; 2DRs, 42%; p=0.000). Switched memory B cells in the 2DR group were higher than those in the BR and NR groups (medians of 30, 17, and 10 cells/ul, respectively; p=0.017). Additionally, the absolute count of central memory/terminal memory CD8 T cells was higher in the 2DR group than in the BR and NR groups. (166, 98, and 93 cells/ul, respectively; p=0.041). The rest of the T-cell populations studied did not show a statistical difference.

Conclusion

switched memory B cells and memory CD8 T-cell populations in peripheral blood were associated with the magnitude of the humoral response after SARS-CoV-2 vaccination. Boosters increased IgG anti-SARS-CoV-2 levels, CM/TM CD8 T cells, and switched MBCs in patients with seropositivity after two doses. Interestingly, no seropositivity after boosters was associated with the use of mycophenolate and a lower number of switched MBCs and CM/TM CD8 T cells in peripheral blood.

Applications of Transcriptomics in the Research of Antibody-Mediated Rejection in Kidney Transplantation: Progress and Perspectives

Antibody-mediated rejection (ABMR) is the major cause of chronic allograft dysfunction and loss in kidney transplantation. The immunological mechanisms of ABMR that have been featured in the latest studies indicate a highly complex interplay between various immune and nonimmune cell types. Clinical diagnostic standards have long been criticized for being arbitrary and the lack of accuracy. Transcriptomic approaches, including microarray and RNA sequencing of allograft biopsies, enable the identification of differential gene expression and the continuous improvement of diagnostics. Given that conventional bulk transcriptomic approaches only reflect the average gene expression but not the status at the single-cell level, thereby ignoring the heterogeneity of the transcriptome across individual cells, single-cell RNA sequencing is rising as a powerful tool to provide a high-resolution transcriptome map of immune cells, which allows the elucidation of the pathogenesis and may facilitate the development of novel strategies for clinical treatment of ABMR.

Investigating the Mechanism of Germinal Center Shutdown

Germinal centers (GCs) are transient structures where affinity maturation of B cells gives rise to high affinity plasma and memory cells. The mechanism of GC shutdown is unclear, despite being an important phenomenon maintaining immune homeostasis. In this study, we used a mathematical model to identify mechanisms that can independently promote contraction of GCs leading to shutdown. We show that GC shutdown can be promoted by antigen consumption by B cells, antigen masking by soluble antibodies, alterations in follicular dendritic cell (FDC) network area, modulation of immune complex cycling rate constants, alterations in T follicular helper signaling, increased terminal differentiation and reduced B cell division capacity. Proposed mechanisms promoted GC contraction by ultimately decreasing the number of B cell divisions and recycling cells. Based on the in-silico predictions, we suggest a combination of experiments that can be potentially employed by future studies to unravel the mechanistic basis of GC shutdown such as measurements of the density of pMHC presentation of B cells, FDC network size per B cell, fraction of cells expressing differentiation markers. We also show that the identified mechanisms differentially affect the efficiency of GC reaction estimated based on the quantity and quality of resulting antibodies.

Current Advances in Follicular Regulatory T-Cell Biology

Follicular regulatory T (TFR) cells are a population of CD4+ T-cells that concomitantly express markers for regulatory T-cells and follicular helper T (TFH) cells, and have been predominantly implicated in the regulation of humoral immunity via their suppressive functions. Rapid and robust progress has been made in the field of TFR cell research since the discovery of this subset over a decade ago. However, there is still a significant gap in our understanding of the mechanisms underlying the phenotypic and functional heterogeneity of TFR cells under various physiologic and pathologic settings. In this review article, we aim to highlight the most up-to-date concepts and investigations in both experimental animal models and human studies to provide a perspective on our understanding of TFR biology with particular emphasis on these cells in the context of disease settings.

The diagnostic role of PD-1+ CXCR5+ follicular helper CD8+ T cell in renal allograft dysfunction

BACKGROUND

The roles of PD-1⁺ CXCR5⁺ follicular helper CD8⁺ T cell were reported in different disease conditions, but their roles in transplantation are unclear. In this study, the association between PD-1⁺ CXCR5⁺ follicular helper CD8⁺ T cell and renal allograft dysfunction in kidney transplant recipients (KTRs) was investigated.

METHODS

82 KTRs were enrolled in this study. 45 KTRs were included in the chronic allograft dysfunction (CAD) group, and 37 KTRs were included in the stable recipients group. Among the CAD group, 12 cases of antibody-mediated rejection (ABMR) and 4 cases of T cell-mediated rejection (TCMR) were diagnosed by biopsy. The percentage of CXCR5⁺ CD8⁺ T cells and the co-expression of signal transducers and activators of transcription 4 (STAT4), STAT5, and PD-1 in peripheral blood were determined by flow cytometry.

RESULTS

The expression of CXCR5 on CD3⁺ CD8⁺ T cells and the percentage of STAT5⁺ CXCR5⁺ cells in the CD3⁺ CD8⁺ T-cell population were significantly lower in the CAD group (p < 0.05), while the expression of PD-1⁺ CXCR5⁺ CD8⁺ T cells was significantly higher (p < 0.05). Through logistic regression analysis, we concluded that the percentage of PD-1⁺ CXCR5⁺ CD8⁺ T cells was an independent risk factor for renal dysfunction. Grouping by pathological type, PD-1⁺ CXCR5⁺ CD8⁺ T cells showed relatively good diagnostic efficacy for ABMR by ROC analysis.

CONCLUSIONS

Our results suggested that PD-1⁺ CXCR5⁺ CD8⁺ T cells were a promising biomarker for distinguishing renal allograft dysfunction and different allograft pathological types. Also, our findings may provide new ways of identifying and treating allograft rejection.

CXCL13 Is an Indicator of Germinal Center Activity and Alloantibody Formation Following Transplantation

Supplemental Digital Content is available in the text.

Donor-specific antibodies (DSA) are a recognized cause of allograft injury, yet biomarkers that indicate their development posttransplant or guide management are not available. CXCL13 (chemokine [C-X-C motif] ligand 1) is a chemoattractant produced within secondary lymphoid organs necessary for germinal center (GC) and alloantibody formation. Perturbations in serum CXCL13 levels have been associated with humoral immune activity. Therefore, CXCL13 may correlate with the formation of HLA antibodies following transplantation.

Targeting T Follicular Helper Cells to Control Humoral Allogeneic Immunity

Humoral allogeneic immunity driven by anti-HLA donor-specific antibodies and antibody-mediated rejection (AMR) significantly impede prolonged survival of organ allografts after transplantation. Although the importance of T follicular helper (TFH) cells in controlling antibody responses has been long established, their role in directing donor-specific antibody generation leading to AMR was only recently appreciated in the clinical setting of organ transplantation. In this review, we provide a comprehensive summary of the current knowledge on the biology of human TFH cells as well as their circulating counterparts and describe their pivotal role in driving humoral alloimmunity. In addition, we discuss the intrinsic effects of current induction therapies and maintenance immunosuppressive drugs as well as of biotherapies on TFH cells and provide future directions and novel opportunities of biotherapeutic targeting of TFH cells that have the potential of bringing the prophylactic and curative treatments of AMR toward personalized and precision medicine.

Personalized Medicine for Kidney Transplantation: Association of Graft Survival and Acute Transplant Rejection with Genetic Variation in B Cell Activating Factor System Signaling

Kidney transplantation (KT) clinical outcomes are highly variable across patients and would benefit from predictive biomarkers to achieve personalized/precision medicine. The B cell activating factor (BAFF) system signaling plays an essential role in B lymphocytes' homeostasis, and is implicated in activation and survival of B lymphocytes. Single nucleotide polymorphisms (SNPs) in BAFF system genes are therefore strong candidates to identify the genetic mechanisms underpinning variable clinical outcomes in KT. We report here new findings on BAFF system genetic polymorphisms in KT patients in relation to two key phenotypes of clinical interest: graft survival and acute rejection (AR). A total of 168 KT patients, of which 29 suffered AR, participated in this study. The BAFF system polymorphisms in five genes TNFSF13B, TNFSF13, TNFRSF13C, TNFRSF13B, and TNFRSF17 were characterized using TaqMan SNP genotyping. Patients with KT who had an AA genotype in polymorphism rs3803800 of the TNFSF13 gene had a higher risk of suffering AR (p = 0.046; odds ratios = 3.38, 95% CI: 1.02-11.2). Moreover, patients with AA genotype (rs3803800) in the TNFSF13 gene had a significantly lower AR-free time than the GG/GA genotypes (69.2% vs. 85.7%; p = 0.037). Of importance, bioinformatics analysis showed that the polymorphism rs3803800 could alter splicing regulation and affect the proliferation-inducing ligand (APRIL) expression levels. The analysis of graft survival did not show a significant association with the polymorphisms analyzed in this study. In conclusion, the rs3803800 genetic polymorphism from this study of BAFF system genes appears to display importance in AR-free time for KT patients, and thus, warrants further research in independent populations as a putative predictive biomarker of AR. These findings also inform future personalized/precision medicine efforts and functional genomic studies in KT patients.

Germinal Centre Shutdown

Germinal Centres (GCs) are transient structures in secondary lymphoid organs, where affinity maturation of B cells takes place following an infection. While GCs are responsible for protective antibody responses, dysregulated GC reactions are associated with autoimmune disease and B cell lymphoma. Typically, ‘normal’ GCs persist for a limited period of time and eventually undergo shutdown. In this review, we focus on an important but unanswered question – what causes the natural termination of the GC reaction? In murine experiments, lack of antigen, absence or constitutive T cell help leads to premature termination of the GC reaction. Consequently, our present understanding is limited to the idea that GCs are terminated due to a decrease in antigen access or changes in the nature of T cell help. However, there is no direct evidence on which biological signals are primarily responsible for natural termination of GCs and a mechanistic understanding is clearly lacking. We discuss the present understanding of the GC shutdown, from factors impacting GC dynamics to changes in cellular interactions/dynamics during the GC lifetime. We also address potential missing links and remaining questions in GC biology, to facilitate further studies to promote a better understanding of GC shutdown in infection and immune dysregulation.

Therapies for Chronic Allograft Rejection

Remarkable advances have been made in the pathophysiology, diagnosis, and treatment of antibody-mediated rejection (ABMR) over the past decades, leading to improved graft outcomes. However, long-term failure is still high and effective treatment for chronic ABMR, an important cause of graft failure, has not yet been identified. Chronic ABMR has a relatively different phenotype from active ABMR and is a slowly progressive disease in which graft injury is mainly caused by de novo donor specific antibodies (DSA). Since most trials of current immunosuppressive therapies for rejection have focused on active ABMR, treatment strategies based on those data might be less effective in chronic ABMR. A better understanding of chronic ABMR may serve as a bridge in establishing treatment strategies to improve graft outcomes. In this in-depth review, we focus on the pathophysiology and characteristics of chronic ABMR along with the newly revised Banff criteria in 2017. In addition, in terms of chronic ABMR, we identify the reasons for the resistance of current immunosuppressive therapies and look at ongoing research that could play a role in setting better treatment strategies in the future. Finally, we review non-invasive biomarkers as tools to monitor for rejection.

Combined Immunotherapy With Belatacept and BTLA Overexpression Attenuates Acute Rejection Following Kidney Transplantation

Background

Costimulatory blockade provides new therapeutic opportunities for ensuring the long-term survival of kidney grafts. The adoption of the novel immunosuppressant Belatacept has been limited, partly due to concerns regarding higher rates and grades of acute rejection in clinical trials. In this study, we hypothesized that a combined therapy, Belatacept combined with BTLA overexpression, may effectively attenuate acute rejection after kidney transplantation.

Materials and Methods

The rat kidney transplantation model was used to investigate graft rejection in single and combined therapy. Graft function was analyzed by detecting serum creatinine. Pathological staining was used to observe histological changes in grafts. The expression of T cells was observed by immunohistochemistry and flow cytometry. In vitro, we constructed an antigen-stimulated immune response by mixed lymphocyte culture, treated with or without Belatacept and BTLA-overexpression adenovirus, to observe the proliferation of receptor cells and the expression of cytokines. In addition, western blot and qRT-PCR analyses were performed to evaluate the expression of CTLA-4 and BTLA at various time points during the immune response.

Results

In rat models, combined therapy reduced the serum creatinine levels and prolonged graft survival compared to single therapy and control groups. Mixed acute rejection was shown in the allogeneic group and inhibited by combination treatment. Belatacept reduced the production of DSA and the deposition of C4d in grafts. Belatacept combined with BTLA overexpression downregulated the secretion of IL-2 and IFN-γ, as well as increasing IL-4 and IL-10 expression. We also found that Belatacept combined with BTLA overexpression inhibited the proliferation of spleen lymphocytes. The duration of the elevated expression levels of CTLA-4 and BTLA differentially affected the immune response.

Conclusion

Belatacept combined with BTLA overexpression attenuated acute rejection after kidney transplantation and prolonged kidney graft survival, which suggests a new approach for the optimization of early immunosuppression after kidney transplantation.

Anti-BAFF Treatment Interferes With Humoral Responses in a Model of Renal Transplantation in Rats

BACKGROUND

B-cell-activating factor (BAFF) is associated with donor-specific antibodies (DSA) and poorer outcomes after renal transplantation (RTx). We examined the effects of anti-BAFF treatment on B cells, expression of costimulatory molecules and cytokines, germinal centers (GCs), and DSA formation in an RTx model in rats.

METHODS

Anti-BAFF antibody was injected on days 3, 17, 31, and 45 after allogeneic RTx. Rats received reduced dose cyclosporine A for 28 or 56 days to allow chronic rejection and DSA formation. Leukocytes, B-cell subsets, and DSA were measured using flow cytometry; expression of cytokines and costimulatory molecules was measured by quantitative polymerase chain reaction, and GCs and T follicular helper were assessed using immunohistochemistry. Rejection was evaluated by a nephropathologist.

RESULTS

Anti-BAFF treatment reduced the frequency of B cells in allografts and spleen. Naive B cells were strongly reduced by anti-BAFF treatment in all compartments. Messenger RNA expression of interleukin-6 and the costimulatory molecules CD40 and inducible T cell costimulator ligand was significantly reduced in anti-BAFF-treated rats. GC area was smaller and plasmablasts/plasma cell numbers lower in anti-BAFF-treated rats, which was reflected by less DSA in certain IgG subclasses.

CONCLUSIONS

Anti-BAFF treatment interferes with humoral responses at multiple levels in this model of allogeneic RTx.

Approaches to inducing antigen-specific immune tolerance in allergy and autoimmunity: Focus on antigen-presenting cells and extracellular vesicles

Increasing prevalence of allergic and autoimmune diseases urges clinicians and researchers to search for new and efficient treatments. Strategies that activate antigen-specific immune tolerance and simultaneously maintain immune reactivity to all other antigens deserve special attention. Accordingly, antigen-presenting cells (APCs) seem to be the best suited for orchestrating these mechanisms by directing T cell immune responses towards a tolerant subtype. Recent advances in understanding cell-to-cell communication via extracellular vesicles (EVs) make the latter promising candidates for reprogramming APCs towards a tolerant phenotype, and for mediating tolerogenic APC function. Thus, comprehensive studies have been undertaken to describe the interactions of APCs and EVs naturally occurring during immune tolerance induction, as well as to develop EV-based manoeuvres enabling the induction of immune tolerance in an antigen-specific manner. In this review, we summarize the findings of relevant studies, with a special emphasis on future perspectives on their translation to clinical practice.

PDL1 blockage increases fetal resorption and Tfr cells but does not affect Tfh/Tfr ratio and B-cell maturation during allogeneic pregnancy

A successful pregnancy requires sophisticated regulation of uterine microenvironment to guarantee the existence of semi-allogeneic conceptus without immune rejection. T follicular regulatory (Tfr) cells exert a suppressive effect on Tfh-cell expansion, B-cell response, and antibody production. Although accumulating evidence has demonstrated that dysregulations of Tfr cells can bring on various immunological diseases, their immunomodulatory roles during pregnancy still remain unheeded. Herein, we introduced an allogeneic normal-pregnant mouse model and found that CD4⁺CXCR5^hiPD-1^hiFoxp3⁺ Tfr cells were preferentially accumulated in the uterus at mid-gestation and displayed a distinct phenotype. In addition, the absence of PDL1 resulted in increased fetal resorption by favoring Tfr cells accumulation and upregulating PD-1 expression on these cells. However, PDL1 blockade affected neither the ratio of Tfh/Tfr cells nor the maturation and differentiation of B cells. Overall, our results are the first to present a correlation of Tfr cells accumulation with healthy allogeneic pregnancy and PDL1 blockade-induced miscarriage, and to indicate that appropriate assembly of Tfr cells is important for pregnancy maintenance. Since blockade of PD-1-PDL1 pathway leads to more Tfr cells and fetal losses, the reproductive safety must be taken into consideration when PD-1/PD-L1 checkpoint blockade immunotherapy is used in pregnancy.

Impact of low tacrolimus exposure and high tacrolimus intra-patient variability on the development of de novo anti-HLA donor-specific antibodies in kidney transplant recipients

Introduction: De novo donor-specific antibodies (dnDSA) directed against HLA are a major contributing factor to the chronic deterioration of renal allograft function. Several factors, including the degree of HLA matching, younger recipient age, and past sensitization events have been shown to increase the risk for the development of dnDSA. The development of dnDSA is also strongly associated with modifications in the immunosuppressive regimen, non-adherence, and under-immunosuppression.Areas covered: Tacrolimus is widely used after solid organ transplantation (SOT) and in recent years, both a high intra-patient variability in tacrolimus exposure and low tacrolimus exposure have been found to be associated with a higher risk of dnDSA development in kidney transplant recipients. This article provides an overview of current findings published in the recent 5 years regarding the relationship between tacrolimus exposure and variation therein and the development of dnDSA.Expert opinion: In this review, we describe how combining data on tacrolimus intra-patient variability and mean pre-dose concentration may be an effective tool to identify kidney transplant recipients who are at higher risk of developing dnDSA.

An overview of T follicular cells in transplantation: spotlight on their clinical significance

Introduction: For late stage organ failure patients, transplantation is the best option to increase life expectancy with a superior quality of life. Unfortunately, after transplantation many patients are at risk of cellular and antibody-mediated rejection (ABMR). The latter is initiated by donor specific antibodies (DSA) which depend on the actions of B cells, T follicular helper (Tfh) cells and T follicular regulatory (Tfr) cells that are present in the germinal center of lymphoid organs.Areas covered: In this overview paper, we discuss the biology and function of Tfh and Tfr cells in lymphoid tissues, transplanted organs and their circulating counterparts. We report on their relevance to alloimmunity and on the effects of immunosuppressive drugs on these immunocompetent cell populations.Expert opinion: Growing knowledge about the actions of Tfh and Tfr allows for a better understanding of the immunological mechanisms of ABMR after organ transplantation. This understanding feeds the hypothesis that immunosuppressive drugs targeting the actions of Tfh cells have huge therapeutic potential. This new concept in the treatment of the humoral rejection response will improve graft and patient survival after organ transplantation.

Follicular helper T cells and humoral response in organ transplantation

Antibody mediated rejection has been recognized as an important contributor to long-term graft loss in most solid organ transplants. Current immunosuppressive regimes are not capable of preventing anti-HLA antibody formation and eventual damage to the graft, and there is a need to develop drugs directed against novel targets to avoid graft allorecognition. In this review we introduce follicular helper T cells (Tfh), a subtype of lymphocyte specialized in helping B cells to differentiate into plasmablasts and produce class-switched antibodies. We focus on the role of Tfh in solid organ transplantation, what is known about Tfh and the production of alloantibodies, how current immunosuppressive therapies affect Tfh and what new molecules could be used to target Tfh in transplantation, with the goal of improving graft survival.